Positive feedbacks from the carbon cycle

Two papers appeared in Geophysical Research Letters today claiming that the warming forecast for the coming century may be underestimated, because of positive feedbacks in the carbon cycle. One comes from Torn and Harte, and the other from Scheffer, Brovkin, and Cox. Both papers conclude that warming in the coming century could be increased by carbon cycle feedbacks, by 25-75% or so. Do we think it’s time to push the big red Stop the Press button down at IPCC?

The approaches of both papers are similar. The covariation of temperature versus CO2 (and methane in Torn and Harte) is tabulated for a record in the past. For the Torn and Harte paper, the time frame chosen is the last 360,000 years, while Scheffer et al. focus on the Little Ice Age, from 1500-1600 A.D. In both cases it is assumed that the climate shift is driven by some external thermal driver. As the temperature warms (in the case of the deglaciation) or cools (the LIA), the CO2 concentration of the atmosphere changes in the sense of a positive feedback, rising associated with warming or falling in response to cooling. The changing CO2 drives a further change in temperature.

In general, it is clear that eventually the sense of these articles could be correct. The response of the terrestrial biosphere to rising CO2 could go either way; toward an increase in uptake because of CO2 fertilization or a longer growing season (as we see today) versus an increase in soil carbon respiration in warmer conditions (the reason why tropical soils contain so little carbon). Uncertainties in the response of the terrestrial biosphere to rising CO2 is a major source of uncertainty for the climate change forecast (Cox et al., 2000).

The oceans are presently taking up about 2 Gton C per year, a significant dent in our emissions of 7 Gton C per year. This could slow in the future, as overturning becomes inhibited by stratification, as the buffer loses its capacity due to acidification. Eventually, the fluxes could reverse as with a decrease in CO2 solubility due to ocean warming.

The biggest question, however, before pushing the Stop the Press button at IPCC, is timing. The CO2 transition through the deglaciation took 10,000 years. (Actually this helps to constrain the cause of the CO2 transition, because the air/sea equilibration time scale for CO2 would be considerably shorter than that.) The timescale that seems intrinsic to IPCC is a century or so, during which we should be able to reap only a small fraction of any harvest that takes 10,000 years to grow. The Scheffer et al paper avoids this issue by restricting its attention to a time period of just a century.

Scheffer et al illustrate the potential feedback for the coming century in a figure which looks something like Figure A.

Temperature depends on CO2 concentration via radiative equilibrium in the blue curves, and CO2 concentration in the air is affected by temperature according to the red lines. A rise in CO2 concentration from an external source changes the equilibrium CO2 as a function of T relation toward higher CO2, to the right, labeled “forcing”. The stable final equilibrium is where the two relations cross, with further CO2 degassing from the land or the ocean, so that more CO2 ends up in the atmosphere than would have if there were no feedback (a vertical red line). A climate sensitivity calculated from the coupled system is higher than one that ignores any carbon cycle feedbacks.

The situation today is complicated somewhat by a carbon spike transient. Atmospheric CO2 is rising so quickly that the terrestrial biosphere and the ocean carbon reservoirs find themselves far out of equilibrium. In attempting to keep up, the other reservoirs are taking up massive amounts of CO2. If emissions were to stop today, it would take a few centuries for the atmosphere to equilibrate, and it would contain something like 25% of our emitted CO2.

I would draw our current situation as in Figure B, with CO2 concentration wildly higher than the equilibrium red line, poised to relax toward lower concentrations if emissions stopped. The effect of the carbon cycle feedback is to change the equilibrium atmospheric CO2 that we are relaxing toward. It seems to me that the most important part of the equation for our immediate future is the decay rate of that carbon spike, rather than the equilibrium value that CO2 will relax to in hundreds of years.

122 comments on this post.
  1. Blair Dowden:

    Re #94: I do not think it is constructive to attack the motives of Geoff Coe. There is a good chance that he is sincere, but has been misled by convincing sounding information. Let me point out a few problems with the “closer look at the numbers” analysis.

    The first line in Table 1 claims 68,520 parts per billion (ppb) of carbon dioxide added to the atmosphere have a “natural” origin. No reference or justification is given. There is no possible natural source for that much carbon dioxide in that period of time. The number is totally false (four significant digits is a good clue as well), which largely invalidates the rest of the analysis which is based on that figure.

    Then in Table 3 we are told that 95% of the greenhouse effect is from water vapor. Again, no justification, just a vague reference to other skeptical web pages. This is based on the fact that about 95% of greenhouse gas by volume is indeed water vapor. It does not take into account that most of the greenhouse effect takes place in the upper atmosphere where water vapor concentrations are much lower. Understanding this takes a bit of work, which I or others here can help you with if you are really interested.

    The actual figures are about two thirds of the greenhouse effect is water vapor, the rest are from other greenhouse gases to which humans have made a significant contribution, a lot more than 0.117%.

    I understand a skeptical outlook. But maybe you should be especially skeptical about people who provide false and misleading information.

  2. Alastair McDonald:

    Re 96 I think your view of farming is rather naive. Their end product is dead. You can’t eat a living steer nor a living field of barley! By producing dead coral we would be locking up carbon dioxide for millions of years, just as is happening in the White Cliffs of Dover. Moreover, if it was spread over the deserts, or elsewhere, it would increase the albedo – two for the price of one!

  3. Geoff Coe:

    You guys need to either restrict this forum to a limited group of accepted members or find a way of addressing new posters without accusing them of having perverted motives. In my opinion, it really doesn’t serve your purpose. In my view, the basic cause of global warming (if human activity is the driving force) is egotism. Egotism sank the Titanic. In my opinion, egotism is really our biggest enemy here. I’d like to know if people here agree with the following newspaper article as far as the earth already being beyond the greenhouse ‘tipping point’…

    February 11, 2006 by the Guardian/UK
    A crucial global warming “tipping point” for the Earth, highlighted only last week by the British Government, has already been passed, with devastating consequences….

    Thank you.

  4. Blair Dowden:

    Re #104: Geoff, I agree that the way your post by some people here was treated was unacceptable. But I did respond with respect, and you did not acknowledge or comment on that. That confirms the suspicions of the posters here – you do not really want to know. Is that correct?

    About the “common dreams” article: I am sick of this kind of hype. The facts given are accurate, but the spin is extreme. There is no “tipping point” (or non-linear effect) that I am aware of in any climate model or plausible climate scenario. Any “threshold” crossed is an arbitrary one. In this case they appear to be comparing a carbon dioxide threshold of 400 ppm, which may be about 50 years in the future, with a carbon dioxide equivalent including other greenhouse gases today. That is the same kind of dishonest accounting used by some skeptics.

    I also have a problem with implied (but not stated) relationship between agricultural yields in Africa and water shortages with global warming. There are many other human caused reasons for these problems. Again, this is dishonest.

    However, again I must point out that the basic facts are correct. Two degrees of warming in this century is a reasonable forecast. Ice cap melting is a serious issue, though more long term than they imply. The basic facts are simple: At any given global average temperature the ice caps contain a certain volume of water, in equilibrium. When it gets colder, ice caps grow and sea levels drop. When it gets warmer the ice caps shrink and sea level rises. We have data from the past on what sea levels were at various temperatures. If you plot a graph of sea level against global temperature, the slope of the line as about 6 meters per degree of warming. So two degrees of warming means 40 feet of sea level rise.

    In equilibrium. Ice does not melt instantly. Equilibrium is reached on the order of 500 to a thousand years. Major effects will be felt sooner than that. See this paper for instance. So the effects of global warming are serious, but will not be felt by the generations responsible for creating them.

  5. Zeke Hausfather:

    Geoff, I’m not entirerly sure what you mean by egotism. Do you mean that an ecotistic and selfish discounting of the future by individuals results in unsustainable behaviors and creates collective action problems like climate change?

    As far as climate change science goes, you have two options as a lay person approaching the field.

    1) Trust the process of peer reviewed science to, in the end, arrive at the correct conclusion. At the moment, the overwhelming majority of the scientific community supports the consensus position embodied the IPCC report (see http://www.sciencemag.org/cgi/content/full/306/5702/1686 for example). While this certainly could change in the future, as new evidence emerges, given the time and effort that has gone into studying climate change at this point, the scientific consensus in itself is probably justification enough for policy makers to take the matter seriously.

    2) Read the relevent science and come to your own conclusions. While this approach is, for obvious reasons, ideally preferable over #1, it comes with its share of pitfalls. First, not every citizen in the world has the time or technical know-how to follow the thousands of relevent research publications every year that help define the field. The reason many of us laypeople trust climate scientists is that we simply don’t have the time to do as detailed a study of the current science as they do. Second, the self-discover risks mistaking cherry-picked data for the overall picture. While websites like CO2Science and their ilk seem rather convincing to the lay audience, an experienced climate scientist can easily pick it appart. Having little time to study the matter fully tends to result in people latching on to the first convincing-looking finding they stumble upon and mistaking it for the “truth”. If you are really interested in this issue, I highly recomend sticking to articles in peer-reviewed publications if possible, as they tend to be less ideologically-tinted and less prone to factual errors or hyperbole.

  6. Lynn Vincentnathan:

    RE #104, I agree with “egotism” being a root cause of AGW. Since GW is being caused by humans, we do need to bring in the social & behavioral sciences to completely understand it, & include the psychological (motivational/cognitive), social, and cultural dimensions.

    Like why in the blank are people dragging their feet on solving this problem? They must be crazy.

    RE the topic, I just read (but have figured all along) that GW may be increasing forest fires (thru drought, early snow melt, wind increase), which would pump more GHGs into the atmosphere — another positive feedback.

  7. Brian Gordon:

    Re: Geoff Coe and it’s too late

    First, my bias has been that I lean toward the “it’s probably too late” camp. This belief, quite honestly, led to some degree of depression and despair. I have kids and even a grandaughter, and I don’t feel very good about the very possible fact that…well…crap, eh?

    Anyway, this negative outlook can be self-reinforcing, because we seek out or believe things that confirm our beliefs; it takes some effort to remain open. However, the ‘doom-and-gloom’ scenario is not backed up by data. Raypierre and Hank Roberts, among others, have helped me realise this. Here’s my reasoning about it:

    1. Even if we stop emitting GHGs immediately, there are enough in the atmosphere to keep warming the earth for some time.
    2. It _appears_ that the Arctic has passed a tipping point, and will soon be ice-free in the summer.
    3. Reputable scientists believe that when the Arctic goes, the melting of the ice on Greenland is inevitable, too. (This from the paper by Hansen et al: http://pubs.giss.nasa.gov/docs/notyet/2005_submitted_Hansen_etal_1.pdf.)
    4. One or both of these events could shut down or drastically slow the THC, freezing Europe.
    5. Who knows what other consequences there would be, but the social disruption might well lead to the collapse of some or all of ‘civilization,’ and therefore the deaths of millions or even billions.

    Sounds rather bleak. However, the authors of the paper referenced do NOT believe that the Arctic is necessarily lost yet, and they state that they believe much of the melting is due to non-CO2 pollution. In addition, we are entering a new era, meaning that some of what happened in the past is not necessarily indicative of what will happen in the future. Will the THC shut down? Will there be massive social disruption? Will Greenland melt? There are lots of unknowns.

    On to the article:
    The Government’s conference on Avoiding Dangerous Climate Change, held at the UK Met Office in Exeter a year ago, highlighted a clear threshold in the accumulation of greenhouse gases such as carbon dioxide (CO2) in the atmosphere, which should not be surpassed if the 2 degree point was to be avoided with “relatively high certainty”.

    This was for the concentration of CO2 and other gases such as methane and nitrous oxide, taken together in their global warming effect, to stay below 400ppm (parts per million) in CO2 terms – or in the jargon, the “equivalent concentration” of CO2 should remain below that level.

    The warning was highlighted in the official report of the Exeter conference, published last week. However, an investigation by The Independent has established that the CO2 equivalent concentration, largely unnoticed by the scientific and political communities, has now risen beyond this threshold.

    This number is not a familiar one even among climate researchers, and is not readily available.

    The 400ppm threshold is based on a paper given at Exeter by Malte Meinhausen of the Swiss Federal Institute of Technology. Dr Meinhausen reviewed a dozen studies of the probability of exceeding the 2 degrees threshold at different CO2 equivalent levels. Taken together they show that only by remaining above 400 is there a very high chance of not doing so.

    As Blair Dowden (#105) said, As Blair Dowden (#105) said,the threshold of 400 ppm is arbitrary. Nobody really knows where the tipping points are, or what each tipping point will ultimately cause. That’s the reason this threshold “is not a familiar one even among climate researchers.” Climate change skeptics use this uncertainty to argue for more study, less action, but that seems a very irresponsible course, given the likely consequences.

    [Finally, I think that “above” should be “below” in the last sentence I quoted from the article.]

  8. Geoff Coe:

    Blair, the reason I didn’t respond to your post was that I didn’t see it before I posted mine (this was because I hadn’t refreshed the web page and didn’t see your posting. Sorry.)

    Zeke, naturally I agree with what you’re saying about the basic two options.

    I would like to throw another link in here for comment if I may…

    When raypierre mentioned “Lomborg” above, I went and looked him up. What is the “consensus” on this particular quote:

    Or to put it more clearly, the temperature that we would have experienced in 2094 [without the Kyoto protical] we have now postponed to 2100….In essence, the Kyoto Protocol…merely buys the world six years [and] will have surprisingly little impact.

    Thank you

    [Response: This quote is typical Lomborg — technically correct, up to a point, but giving a completely misleading impression. The assumption behind the quote is that the Kyoto Protocol is the only thing the world is going to do about global warming — that we’ll do Kyoto (if that) and then call it quits. In reality, Kyoto is just the first step in a very long process which will eventuallly involve all the world’s CO2 producers and not just the Annex I countries. Kyoto provides some impetus to develop the right technologies which can be transferred or sold to the developing world, and provides some real-world experience with the cost of CO2 abatement. It also provides the necessary moral platform for involving the developing world. If the Annex I countries can’t even commit to the modest first steps in Kyoto, they have no moral standing to demand stronger actions from poorer countries who have not benefited as much so far from industrial growth. One could quite reasonably argue whether Kyoto is the most cost-effective way to take the first step. One could not argue that the modest climate benefits accrued directly from the Kyoto reductions are justification for doing nothing at all. If anything, this is an argument for finding a way to do more, faster.

    Instead of changing the subject by tossing out yet another standard skeptics’ quote, it would be really nice if you would tell us what you think of the scientific arguments given in the mainstream literature links that various people suggested you read. –raypierre]

  9. Geoff Coe:

    Raypierre, I find your reply to the Lomborg quote very depressing because, assuming that you’re right (and I’m not assuming that you’re not) the situation is impossible (or at least appears that way to me at the moment.)

    Did you know, for example, that 44% of the American population now believes that Jesus will return to earth to judge the living and the dead sometime in the next fifty years (source here and here). I mention this because these people do not see a need to deal with the “problem” of global warming. How could global warming be a problem if they’re just about to be drawn up to Heaven in the Rapture? 44%! That’s a lot! Practically half. From what I’ve gathered (see sources above), these people view any attempt to address global warming as a form of blasphemy because it is somehow an attempt to circumvent the Will of God, etc. It’s sickening, but this is part of what I meant when I said before that I see anthropogenic global warming as a function of ego (ego being the tendency to be stuck in a limited point of view.) At the same time, however, I’m not saying that we shouldn’t take action wherever we see a chance to effect a positive change. We maximize our ability to make a positive difference by having a clear picture of what we’re up against. Given that number (44%) it’s a waste of time to approach the situation with the assumption that most people are approaching the situation from the same pragmatic place that you are.

  10. Florifulgurator:

    Geoff, perhaps my apologies are due (for #100)…
    Yet, I´m not sure you´re not trying to propagate some sort of “3rd-order denialism” (i.e.: “we can´t do anything because people don´t want to do anything”).

    Your 44% number is one reason why I chose to adress certain people like I did with you. This is anno domini 2006. Forget the 20th century – things will get worse this round. People who still don´t get it (AGW) are either 1) brainwashed 2) in psychopathic denial of reality 3) corrupted 4) religiously deluded or 5) I dunno. It is time to tell that straight into their face. Sorry.

  11. Coby:


    Some have said you were treated harshly, and it seems you have decided to agree. Myself, I concede I was curt, but I don’t think anything I said in tone or content was rude, it certainly wasn’t intended this way. Raypierre’s initial response could be seen as brusque, though I’m sure he means no offense either and, like me, is merely frustrated with a recurring pattern and the never ending citations of demonstrable junk (granted, maybe unkowingly cited).

    But you are currently failing the next test of what your real motives are. You do not respond to the offers of quality information and you abandon the subjects of your initial enquiries without conceding anything only to offer up new, and typical, septic talking points.

    Please don’t be offended if you are still sincere, this maybe your first foray into a public discussion of this issue so you don’t know the changing climate of climate change conversations, but some of us have seen a sad occurence time and time again of someone posing as a naive and sincere concerned citizen once trusting, now befuddled by conflicting scientific views only to eventually reveal that their minds were long ago firmly made up and their purpose is merely to troll and muddy the waters for everyone.

    If you are sincere, please let us know that you read and understood what we offered as references rather than continuing to offer your own. If you are not, you will soon claim we are all religious zealots who can not defend our positions logically so only attack and belittle those true denialist heros who are not afraid to cry out the emperor has no clothes.

    Who are you, Geoff Coe?

    [Response: I was brusque firstly because I had already seen enough posts worded very similarly to Geoff Coe’s to make me suspicious of the motives. I was also brusque because I find it hard to believe that, with all the valid information out there, with the well-written IPCC reports written by several hundred top scientists carefully screened for their scientific qualifications and carefully reviewed by governments and other scientists, with statements on global warming by the American Meteorological Society, American Geophysical Union, most of the world’s National Academies, with scientific research conducted by or endorsed by members of the National Academy of Sciences, MacArthur Award winners, Fellows of the Royal Society, Nobel prize winners etc. etc. — with all this out there it strains belief that anybody who was sincerely interested in seeking knowledge would light on two of the least reliable sources in the whole climate change universe and pin his beliefs on these without even doing a cursory check of what the scientific community thinks about the claims made in these sources. In light of this, I really wasn’t expecting much, but I was curious to see just how this phenomenon happens, if it is indeed a sincere request for clarification. I do appreciate that Mr. Coe didn’t just cut and run (which usually happens in these cases), but I am still awaiting enlightenment. And, I’m still awaiting any sign that Mr. Coe has read and considered the suggested material. –raypierre]

  12. Joel Shore:

    Re #104: Not to nitpick too much, but with CO2 levels currently passing through ~380ppm and growing at about 0.5%…or 2ppm…per year, we will be crossing through 400ppm of CO2 in about 10 years, not 50.

    Re #108 (Lomborg’s quote about Kyoto): Another way to look at it, in addition to what raypierre said, is simply this — Kyoto proscribes emissions for the developed (Annex I) nations for a 5-year period (2008-2012). So, even if Kyoto applied to all nations and involved producing no emissions at all for a five-year period (after which we went back to our normal ways), the net effect would only be to delay the warming effects by ~5 years! Given the numbers that Lomborg comes up with (and knowing that the actual emission cuts proscribed are not nearly so draconian), I would assume that Lomborg’s claim involves somewhat more reasonable assumptions about what happens to emissions outside of the 5 year 2008-2012 window, but this does give you the basic idea: It is silly to make statements like this about what a treaty that proscribes emissions for a 5-year time period will accomplish without at least carefully explaining your assumptions. I have seen these sort of statements made about Kyoto constantly and have yet to see any place where they are stated with the underlying assumptions carefully spelled out. Without those, the statements are, in my view, completely vacuous.

    The way I think of Kyoto is that its major purpose is to correct the current market failing by which the cost of CO2 emissions is “externalized” so that each person can use the atmosphere as a free sewer and then we collectively pay for everyone else’s emissions. Under such a system, there are no market incentives pushing the development and implementation of technologies that reduce or sequester our greenhouse gas emissions. In order for the market to have the right incentives, it is necessary to “internalize” the cost on those emissions, which is what Kyoto effectively does (especially under the emissions trading regime that has been set up).

    One of the ironies is that those who sometimes claim to be the strongest believers in markets and claim that markets will somehow solve the problem without any intervention are either ignorant about how markets actually work or are liars. They are actually hoping that magic…or at least extra-market forces such as altruism…will lead people to solve the problem because the market will not solve problems where the costs are externalized. It simply makes no sense for someone to invest money in technology to solve a problem that they are not paying for. (Of course, we are all collectively paying for our emissions through global warming and other environmental problems, but nearly all of the costs each of us bears is due to the emissions of everyone else. Only a vanishingly small amount is due to our own emissions.)

  13. Geoff Coe:

    Coby wrote: “Who are you, Geoff Coe?”
    I wrote a reply to this but then felt uncomfortable about posting stuff about myself up here. For those who want to read what I wrote, click here. I understand the fear of having one’s time wasted by someone who, in the end, may not be sincere and may have already formed an undisclosed opinion, but I really don’t know what I can do at this point to address that fear. I think that everyone must naturally make up his or her own mind. Besides what I wrote on the other page, I’m sort of at a loss to know what would be the right or proper answer.

  14. Blair Dowden:

    Re #112: Joel, of course you are correct about carbon dioxide levels. I guess the Common Dreams article threw so many arbitrary numbers around that I got confused myself. Setting 400 ppm as a magic barrier when we are almost already there makes no sense.

    I think the only way to rational market decisions about fossil fuel use is to place a carbon tax on them at the source, although I realize there are problems on how to handle the revenue. I am not sure how this fits in with the Kyoto emissions trading scheme.

    Re #113: Geoff, I believed you from the start. My only suggestion is to follow through on your questions before changing the topic.

  15. Coby:

    Hi Geoff,

    To address that fear, just let me know if you understand the deceit in the clearlight pages when it comes to H2O vs CO2 in climate change. Specifically, did you read and do you follow the arguments and information in these three links?

    Are you reassured that, Bill Gray’s contemptuous laughter aside, there is in fact a very strong scientific consensus that GW is real and CO2 emissions are the primary driver?
    (Bill Gray is in a *very* small majority even in the sceptic world by denying the above)

    And have you had time to look into this document?

    A sincere interest can not be better served than by reading at least the Summary for Policy Makers in the above IPCC report, and hopefully you would have time to read summaries and introductions of the various chapters as well.

    Best wishes,

  16. Coby:

    (Bill Gray is in a *very* small majority even in the sceptic world by denying the above)

    should have been “small minority” of course…

  17. Geoff Coe:

    Coby: I am reading/looking at the pages that you posted. In general, I want people to understand that, looking back at my first post (#94), I can understand now why people here would think that I might not be entering the situation with a fully open mind. Chalk it up to being unaware of the negative experiences that people on this list have been through. Please be clear that my central interest here is the World Peace Society Project and learning that will assist me in moving that project forward. Given the extreme gravity of the global warming issue, it’s clearly important that I be informed about it. From a conventional point of view, however, I think that my approach to learning is bound to look a bit chaotic or scattered at times. You should know, however, that every link or reference given to me by people from this site has either been bookmarked or recorded. I wouldn’t want anyone here to feel that their efforts with me were wasted. World peace does boil down to trust and I am satisfied that the thinkers on this page are both diligent and sincere. At this point, therefore, I don’t understand all the science. In my opinion, however, if there’s a large group of diligent and thoughtful individuals who believe that global warming is being driven by human CO2 emissions, then the proper and moral thing to do is to assume that’s the truth until it’s been thoroughly disproven. To do anything else is playing Russian Roulette with the world. So please don’t get angry at me if I don’t end up grasping all the intricacies of climate science. I am interested. How much I’ll learn and when, however, will depend on a number of different factors. I’d like to suggest, however, that even when you’re speaking with someone who isn’t sincere, you’re sincere efforts always make a difference, even if it’s not the particular difference that you had in mind. That’s my suggestion.

  18. John L. McCormick:

    Mr. Coe,

    You have much to read and I do hope you take some time off, some serious time off to self-educate just like many of us non-scientists have had to do. Then, maybe this thread can move on.

    Your comment below says volumes about the depth to which you seem to be plowing into the many informative and scientific sites provided by thoughtful individuals with the sincere hope you will read them; i.e., the IPCC Third Review to name just one.

    “In my opinion, however, if there’s a large group of diligent and thoughtful individuals who believe that global warming is being driven by human CO2 emissions, then the proper and moral thing to do is to assume that’s the truth until it’s been thoroughly disproven.”

    The train is way down the track and it aint slowing down.

  19. Coby:

    In my opinion, however, if there’s a large group of diligent and thoughtful individuals who believe that global warming is being driven by human CO2 emissions, then the proper and moral thing to do is to assume that’s the truth until it’s been thoroughly disproven.

    Nicely put and I heartily agree. It is refreshing to hear someone in the early learning phase, such as yourself, understand that, rather than take the opposite view: until we hear the hull buckling on the rocks, full speed ahead – which is what the “prove it beyond a doubt first” attitude boils to.

  20. Blair Dowden:

    To return to the original topic, the June 16 issue of Science contains the article Permafrost and the Global Carbon Budget, which claims that previous estimates of carbon in Arctic permafrost are too low. They suggest that 500 gigatonnes (Gt) carbon are tied up in “yedoma” (frozen loess) and another 400 Gt in other permafrost (compared to 730 Gt in the air today). After it melts, they say most of the carbon is released within a century. In what they call the “extreme scenario” where all the tundra melts during this century, carbon dioxide levels could double from this source alone.

    I would like to know how different this information is than what is currently believed, and level of uncertainty you think there is.

    [Response:I had seen comparable numbers to this for carbon inventories in Arctic permafrost, but this is a more reliable-looking (more detailed) treatment than I’d seen before (an old paper by MacDonald who pulls a number out of the air, is the paper everyone seems to cite). I have to say that the second-to-last paragraph in the paper, about carbon isotopes and reservoir changes during glacial time, seems a bit garbled to me. The sizable marine organic carbon reservoir is dissolved organic matter, which is today thousands of years old (by radiocarbon). It’s not clear what determines the size of this reservoir, nor is it clear from the text that this is what the authors are referring to anyway. But for the future, the prediction that melting soils would release carbon, relatively quickly, and potentially a lot of it, that conclusion seems robust to me. David]

  21. Blair Dowden:

    Thank you, David. If I understand you correctly, this paper confirms our present understanding, rather than revising the impact of melting permafrost upward. The paper does not give a ratio of methane to carbon dioxide for emissons from melting permafrost, but I assume the methane portion is significant. Since melting of permafrost has already begun, one would expect it to affect methane concentration. But methane levels seem to have levelled off, at least for the past few years. The missing piece of information is how much permafrost has melted, and what portion of its carbon has been released.

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